core/unit: fix log message
    
    As you can see in the below, the dropped dependency Before=issue-24990.service
    is not logged, but the dependency Before=test1.service which is not owned by
    the units generated by the TEST-26 is logged.
    
    Before:
    systemd[1]: issue-24990.service: Dependency After=test1.service dropped, merged into issue-24990.service
    systemd[1]: issue-24990.service: Dependency Before=test1.service dropped, merged into issue-24990.service
    
    After:
    systemd[1]: issue-24990.service: Dependency After=test1.service is dropped, as test1.service is merged into issue-24990.service.
    systemd[1]: issue-24990.service: Dependency Before=issue-24990.service in test1.service is dropped, as test1.service is merged into issue-24990.service.
    
    (cherry picked from commit ed9911630e4bca844381d7caeb850dad9a9fa122)